Brian Steel is a Tesla believer. The auto mechanic has put nearly $250,000 into Tesla Motors Inc. stock, ignoring warnings from his financial adviser. As the stock price soared, his profits even allowed him to buy a Tesla Model S.

"Tesla is going to change the world," Mr. Steel, 62 years old, of Tacoma, Wash., said of the upstart electric-car maker, whose rocketing stock rally and screeching pullback have been the talk of...